Charged particle beam columns are typically employed in scanning electron microscopy (SEM), which is a known technique widely used in the manufacture of semiconductor wafers, being utilized in a CD metrology tool, the so-called CD-SEM (critical dimension scanning electron microscope), and a defect review SEM (DR-SEM).
In SEM, the region of a sample to be examined is two-dimensionally scanned by means of a focused primary beam of electrically charged particles, usually electrons. Irradiation of the sample with the primary electron beam releases secondary (and/or backscattered) electrons. The secondary electrons are released at that side of the sample at which the primary electron beam is incident, and move back to be captured by a detector, which generates an output electric signal proportional to the so-detected electric current. The energy and/or the energy distribution of the secondary electrons is indicative of the nature and composition of the sample.
Various CD-SEMs and method for measuring critical dimensions are illustrated in the following U.S. patent applications which are incorporated herein by reference: U.S. patent application publication number 2003/0015699 of Su, titled “Integrated critical dimension control for semiconductor device manufacturing”; U.S. patent application publication number 2005/0048654 of Wu, titled “Method of evaluating reticle pattern overlay registration”; U.S. patent application publication number 2004/0173746 of Petrov, et al., titled “Method and system for use in the monitoring of samples with a charged particles beam”; U.S. patent application publication number 2004/0056207 of Petrov, et al., titled “Deflection method and system for use in a charged particle beam column”; U.S. patent application publication number 2003/0218133 of Petrov, et al., titled “Charged particle beam column and method for directing a charged particle beam”, and U.S. patent application publication number 2003/0209667 of Petrov, et al., titled “Charged particle beam apparatus and method for inspecting samples.
An SEM includes such main constructional parts as an electron beam source (formed with a small tip called an “electron gun”), an electron beam column, and a detector unit. The detector unit may be located outside the propagation path of the primary beam through the column, or may be located in the path of the primary beam (the so-called “in-column” or “in-lens” detector). The electron beam column includes, inter alia, a beam focusing/deflecting arrangement formed by a lens assembly and a deflector assembly. The deflection of the primary beam provides for scanning the beam within a scan area on the sample, and also for adjusting incidence of the primary beam onto the sample (an angle of incidence and/or beam shift), as well as directing the secondary beam to the detector.
In order to obtain high resolution, the SEM scans the sample with a very narrow spot while using very high acceleration voltages. Specifically, the electron optical elements are more effective (i.e., produce smaller aberrations) when the electrons are accelerated to high kinetic energy. Scanning the sample with such a narrow spot causes the sample to be charged along the scan path of the charge particle beam.
Preciseness of measurements, such as CD measurements on semiconductor wafers and especially on lithographic masks (reticles), typically suffers from this negative charging of the sample's surface by the scanning beam of charged particles, which causes an image drift. U.S. Pat. No. 6,555,815, assigned to the assignee of the present application, discloses a charged particle beam column where charging of the specimen is avoided or reduced by injecting inert gas onto the sample's surface.